Sputtering target, process for its production and film forming method

ABSTRACT

A sputtering target which comprises SiC and metallic Si and has an atomic ratio of C to Si of from 0.5 to 0.95 and a density of from 2.75×10 3  kg/m 3  to 3.1×10 3  kg/m 3  and which is capable of forming a film comprising SiO 2  as the main component and having a low refractive index at a high speed; a process for its production; and a film-forming method.

DESCRIPTION

[0001] 1. Technical Field

[0002] The present invention relates to a target to be used for forminga film comprising SiO₂ as the main component by a sputtering method, aprocess for its production, and a film-forming method for a filmcomprising SiO₂ as the main component by means of such a target.

[0003] 2. Background Art

[0004] Heretofore, as a material for a low refractive index film(refractive index n<1.6), SiO₂ (n=1.46), MgF₂ (n=1.38) or the like, hasbeen known. Such a film material can be formed into a film by a vacuumdeposition method or a wet-coating method, but a sputtering method isemployed in many cases when a film is formed on a substrate of a largearea, such as glass for buildings, glass for automobiles, a cathode raytube (CRT) or a flat display. Among sputtering methods, a direct current(DC) sputtering method utilizing direct current discharge, isparticularly suitable for film forming over a large area. Heretofore, ithas been difficult to form a SiO₂ film having high mechanical durabilityby a sputtering method in an atmosphere containing oxygen (a so-calledreactive sputtering method) employing a Si target, because abnormaldischarge (arcing) takes place. In recent years, a technique to suppressarcing has been developed, such as an improvement of the film-formingapparatus, whereby forming of a SiO₂ film by a reactive sputteringmethod has been practically carried out, but the film-forming speed isnot yet adequate. Further, a polycrystal Si target or a single crystalSi target to be used has had a problem that it is susceptible tocracking along grain boundaries or crystal faces. In order to make ithardly susceptible to cracking, a Si target having Al incorporated, hasbeen proposed (JP-A-5-501587) but as Al is incorporated as an impurityinto the SiO₂ film, there has been a problem that the refractive indexof the film increases. Further, there has been another problem that thefilm-forming speed is slow.

[0005] Further, it has been proposed to use a sintered SiC target toform a Si_(X)O_(Y)C_(Z) film (JP-A-63-113507), but, when the sinteredSiC target is employed, the film-forming speed has not been adequate.

[0006] It is an object of the present invention to provide a targetwhereby a film comprising SiO₂ as the main component and having a lowrefractive index, can be formed at a high speed by a sputtering method,a process for its production, and a film forming method for a filmcomprising SiO₂ as the main component by means of such a target.

[0007] Another object of the present invention is to provide a targetwhereby a film comprising SiO₂ as the main component and having a lowrefractive index, can be formed at a high speed by a sputtering method,and the durability against cracking during film forming is improved, aprocess for its production, and a film-forming method for a filmcomprising SiO₂ as the main component by means of such a target.

DISCLOSURE OF THE INVENTION

[0008] The present invention provides a sputtering target whichcomprises SiC and metallic Si and which has an atomic ratio of C to Siof from 0.5 to 0.95 and a density of from 2.75×10³ kg/m³ to 3.1×10³kg/m³.

BEST MODE FOR CARRYING OUT THE INVENTION

[0009] In the target of the present invention, the atomic ratio of C toSi (the sum of Si in SiC and Si in metallic Si), i.e. C/Si (atomicratio), is from 0.5 to 0.95. The metallic Si in the present invention ismeant for commonly known Si having a nature of a semiconductor.

[0010] If C/Si (atomic ratio) in the target of the present invention isless than 0.5, the Si amount tends to be large, and the film-formingspeed tends to decrease. If it exceeds 0.95, the film-forming speedlikewise tends to decrease. For example, when C/Si (atomic ratio) is1.0, i.e. in the case of SiC, the film-forming speed is low as comparedwith in the case of a target of the present invention comprising SiC andmetallic Si as the main components. C/Si (atomic ratio) is particularlypreferably from 0.7 to 0.9.

[0011] In a case where C/Si (atomic ratio) is within a range of from 0.5to 0.95, if the density is less than 2.75×10³ kg/m³, the discharge tendsto be instable, and if it exceeds 3.1×10³ kg/m³, the film-forming speedtends to decrease.

[0012] The target of the present invention comprises SiC and metallicSi, as the main components, whereby cracking along grain boundaries oralong cleavage planes of Si particles which used to be a cause forcracking, will be suppressed by a phase of SiC. Further, the chemicalbonding strength between Si and C in SiC is strong, whereby the targetis scarcely susceptible to cracking even if a large electric power isapplied.

[0013] In the target of the present invention, the metallic Si ispreferably present to fill spaces among SiC particles and to constitutea continuous body from the viewpoint of the resistivity, dischargestability and thermal conductivity.

[0014] The thermal conductivity of the target of the present inventionis preferably at least 100 W/(m·K). If the thermal conductivity is lessthan 100 W/(m·K), the target tends to have a high temperature locally,whereby the target tends to have damages such as cracks. Further, due toa local high temperature of the target, such a portion tends to besusceptible to oxidation and thus is likely to cause deterioration ofthe film-forming speed. The higher the thermal conductivity, the better.However, if it exceeds 200 W/(m·K) there will be no difference in theeffects of suppressing local temperature rise.

[0015] In the target of the present invention, the total of impurities(components other than Si and C) is preferably at most 1 mass %, basedon the total amount of the target, with a view to obtaining a filmcomposed mainly of a SiO₂ film having a low refractive index.

[0016] The target of the present invention comprises SiC and metallic Sias the main components, whereby as compared with conventional Sitargets, the film-forming speed can be made large per unit appliedelectric power. It is considered that as compared with Si, with SiC,generation of secondary electrons is low, and the sputtering currenttends to be low, whereby the voltage tends to be relatively high, andthe sputtering efficiency is thereby improved.

[0017] The relative density of the target of the present invention ispreferably at least 60% from the viewpoint of the stability of dischargeduring the film forming. Further, the resistivity of the target of thepresent invention is preferably at most 0.5 Ω·m from the viewpoint ofcarrying out DC sputtering. Further, it is preferably at most 0.03 Ω·mfrom the viewpoint of the stability of discharge.

[0018] The target of the present invention is useful also for RF (radiofrequency) sputtering.

[0019] As compared with other targets comprising SiC as the maincomponent, the target of the present invention is excellent inelectrical conductivity, whereby electric discharge machining ispossible, and the plasma will be stabilized during sputtering discharge.Further, the target of the present invention can easily be processed bymechanical processing.

[0020] The target of the present invention is prepared, for example, asfollows.

[0021] To SiC powder, a dispersing agent, a binder (such as an organicbinder) and water are adjusted and added, followed by stirring to obtaina slurry of SiC. Then, this slurry is introduced into a gypsum mold andmolded by casting. After through drying, the molded product is obtainedby removing it from the mold.

[0022] The casting method is an industrially useful molding method whichis inexpensive and provides high productivity and whereby a producthaving a large area or an irregular shaped product other than a flatplate can also be formed.

[0023] In the above example, the molded product is obtained by a castingmethod, but a press molding method or an extrusion molding method mayalso be used. Further, with respect to the shape of the molded product,a desired shape such as a plate shape or a cylindrical shape maysuitably be selected.

[0024] The molded product thus obtained, may be dried, as the caserequires. Further, in a case where a sintered product is to be obtainedfrom the molded product, the molded product is fired at a temperature offrom 1450° C. to 2300° C. in vacuo or in a non-oxidizing atmosphere, toobtain a sintered product. The sintering temperature is preferably from1500° C. to 2200° C., particularly preferably from 1600° C. to 1800° C.,whereby pores will be formed which are suitable for the next step ofimpregnation with molten Si.

[0025] The obtained molded product or sintered product is impregnatedwith molten metal Si at a temperature of from 1450° C. to 2200° C. invacuo or in a reduced pressure non-oxidizing atmosphere, to have poresof the molded product or sintered product filled with the metallic Si,to obtain a target. With a view to suppressing the evaporation amount ofSi while promoting impregnation of Si, it is preferably from 1500° C. to2200° C., particularly preferably from 1500° C. to 1800° C.

[0026] As conventional methods for producing SiC targets, a pressurelesssintering method, a hot pressing method and a reaction sintering methodare, for example, known.

[0027] However, in the pressureless sintering method, an impurity isusually involved as a sintering aid, and shrinkage takes place bysintering, whereby a residual stress will remain in the sinteredproduct, which is likely to cause cracking of the target.

[0028] By the hot pressing method, it is difficult to obtain a sinteredproduct having a large area, and, like the above-mentioned pressurelesssintering method, a residual strain will remain in the sintered product,which is likely to cause cracking of the target. Further, after hotpressing, processing such as scraping is required, and the method is notefficient. Further, the hot pressing method is not suitable for theproduction of a cylindrical shape or a complicated shape.

[0029] The reaction sintering method is a method wherein a moldedproduct is obtained by using a SiC powder and a carbon powder as rawmaterials, and then the carbon in the molded product and impregnatedmolten Si are reacted. By such a method, a volume expansion takes placeat the time of the reaction of the carbon with molten Si, andconsequently, a residual stress will remain, which is likely to causecracking of the target. Further, it is difficult to obtain a slurrywherein the SiC powder and the carbon powder used as raw materials, areuniformly mixed, whereby a casting method industrially useful can hardlybe employed. For such a reason, in the present invention wherein moltenSi is impregnated, it is preferred not to use carbon powder as a rawmaterial.

[0030] In the process for producing the target of the present invention,the method of impregnating the molded product with molten Si withoutsintering the molded product is superior in the productivity, since thesintering step is omitted.

[0031] Further, the method of impregnating it with molten Si afterfiring the molded product to obtain a sintered product, has a merit inthat impurities will be evaporated in the sintering step, whereby atarget of higher purity can be obtained.

[0032] The target of the present invention is usually processed into apredetermined size and then 1) bonded to a backing plate made of metalby a bonding material (such as indium), or 2) mechanically fixed to acathode by a jig such as a cramp and then used.

[0033] The present invention further provides a film-forming methodwhich comprises forming a film comprising SiO₂ as the main component inan atmosphere containing an oxidizing gas by a sputtering method,characterized in that as a sputtering target, the above-describedsputtering target is used. As the sputtering method in the presentinvention, a sputtering method capable of high speed film forming may bementioned. Specifically, a DC sputtering method, a sputtering methodwith a frequency lower than the frequency in the RF sputtering method,or a sputtering method having the waveform of the appliedcurrent/applied voltage in DC sputtering changed (for example, changedinto a rectangular shape), may be mentioned. Each of such sputteringmethods is a sputtering method requiring an electrically conductivetarget.

[0034] In the present invention, the film comprising SiO₂ as the maincomponent (hereinafter referred to simply as a SiO₂ film) is preferablysuch that the SiO₂ component is at least 99 mass %, based on the totalamount of the film. The refractive index at a wavelength of 633 nm ofthe Sio² film in the present invention, is preferably at most 1.50,particularly preferably at most 1.48.

[0035] The SiO₂ film in the present invention preferably contains a Ccomponent slightly or contains substantially no C component. If C is 0.2mass % or less, based on the total amount of the film, a SiO₂ filmhaving a low refractive index, which shows no substantial absorption oflight, will be obtained.

[0036] In order to form the SiO₂ film at a high speed, it is importantthat 1) a large electric power can be applied to the target, and 2) thefilm-forming speed per unit applied electric power, can be made high.

[0037] As the applied electric power, it is preferred that the powerdensity against the target (one obtained by dividing the maximum powerby the area on the discharge surface side of the target) is at least 10W/cm², with a view to obtaining the SiO₂ film at a high speed.

[0038] In reactive sputtering in an atmosphere containing an oxidizinggas, the target surface is oxidized, whereby a stress is likely to formbetween the surface and the interior of the target, whereby the targetis likely to undergo cracking. However, SiC will suppress oxidation ofthe target surface, and, as mentioned above, it has high resistanceagainst cracking, whereby cracking is less likely to occur even in suchreactive sputtering.

[0039] In the film-forming method of the present invention, an oxidizinggas (such as O₂ gas or CO₂ gas) is contained in the sputteringatmosphere (i.e. the sputtering gas) during film forming by sputtering.By adjusting the partial pressure of the oxidizing gas during filmforming by sputtering, the C component of SiC in the target can beprevented from being taken into the film, whereby a SiO₂ film free fromimpurity can be obtained.

[0040] In the film-forming method of the present invention, the Ccomponent of SiC in the target will be reacted with the oxidizing gas(particularly preferably the O₂ gas) in the atmosphere during filmforming to form CO₂ or CO, which is then discharged by a vacuum pump.

[0041] In the present invention, the SiO₂ film is formed, for example,as follows.

[0042] The target of the present invention is mounted on a magnetron DCsputtering apparatus. Then, the film-forming chamber is evacuated, andthen, as a sputtering gas, a mixed gas of Ar gas and O₂ gas, isintroduced. The proportion of the O₂ gas in the sputtering gas ispreferably from 20 volume % to 100 volume %. If it is lower than 20volume %, oxidation tends to be inadequate, whereby a non-absorptiveSiO₂ film tends to be hardly obtainable. From the viewpoint of thefilm-forming speed, it is preferably at most 80 volume %. Especially,the proportion of the O₂ gas in the sputtering gas is preferably from 30volume % to 80 volume %.

[0043] In the present invention, it is preferred to apply a voltage of apulsed waveform to the target in order to suppress arcing during filmforming.

[0044] The substrate on which the SiO₂ film is formed, is notparticularly limited, and a glass substrate, a plastic substrate or aplastic film may, for example, be mentioned.

[0045] The thickness of the SiO₂ film (the geometrical film thickness)is preferably from 10 nm to 1 μm from the viewpoint of using as anantireflection film.

EXAMPLE 1

[0046] A dispersing agent was added to a SiC powder and mixed in a ballmill using distilled water as a medium. Then, a binder was furtheradded, followed by stirring to obtain a slurry for casting. The slurrywas introduced into a gypsum mold, and casting was carried out. Afterdrying, the molded product was removed from the mold and furtherthoroughly dried. Then, it was held for two hours at 1600° C. to carryout firing in vacuo while evacuating by means of a vacuum pump. Thedensity of the obtained sintered product was 2.6×10³ kg/m³ (relativedensity: about 81%). This sintered product was immersed in metallic Simelted at 1600° C. in vacuo to have metallic Si impregnated to obtain atarget comprising SiC and metallic Si as the main components.

[0047] The density of the obtained target was 3.0×10³ kg/m³ (relativedensity: about 100%). The resistivity of the target was 1.2×10³ Ω·m.C/Si (atomic ratio) of this target was 0.8. The thermal conductivity ofthe target as measured by a laser flash method, was 150 W/(m·K).Further, the target was subjected to an X-ray diffraction analysis,whereby only crystal phases of SiC and Si were observed. Further, it wasconfirmed that metallic Si was present to fill spaces among SiCparticles and to constitute a continuous body.

[0048] Further, by ICP (inductively coupled plasma emissionspectrometry), the amounts of metal impurities based on the total amountof the target were measured, whereby Al was 0.01 mass %, Fe was 0.005mass %, Ti was 0.002 mass %, Ca was 0.001 mass %, Mg was less than 0.001mass %, V is 0.003 mass %, Cr was less than 0.001 mass %, Mn was 0.002mass %, and Ni was less than 0.001 mass %.

[0049] The obtained target was subjected to electric discharge machiningand grinding to a size of 150 mm in diameter and 15 mm in thickness andbonded to a backing plate made of copper by a metal bond. This targetwas mounted on a magnetron DC sputtering apparatus, and film forming wascarried out. The grinding at that time was easy. The conditions duringfilm forming were such that the back pressure was 1.3×10⁻³ Pa, and thesputtering pressure was 0.4 Pa. As the sputtering gas, a mixed gas of Arand O₂ wherein the proportion of O₂ gas in the sputtering gas was 50volume %, was employed.

[0050] Further, the applied electric power was 3 kW, and the electricpower was applied so that the voltage would be a rectangular wave. Thepower density at that time was 17 W/cm². When the time during which aminus voltage was applied, is represented by ON time, and the timeduring which a plus voltage was applied, is represented by OFF time, theON time was set to be 50×10⁻⁶ sec, and the OFF time was set to be50×10⁻⁶ sec. At that time, the voltage during the ON time was −720V.Further, the voltage during the OFF time was set to be +50V.

[0051] As the substrate, a soda lime glass substrate was used. To thesubstrate, no intentional heating was applied. The operation was carriedout so that the film thickness would be about 500 nm. The dischargeduring sputtering was very stable, and film forming was carried outstably even by DC sputtering.

[0052] After film forming, the film thickness was measured by means of afilm thickness measuring apparatus of feeler type. The film-formingspeed per unit applied electric power was 120 nm/(min·kW), and thefilm-forming speed was 360 nm/min.

[0053] The refractive index of the film was measured by an ellipsometer. The wavelength of light used was 633 nm (measured in the samemanner in other Examples). The refractive index of the film was 1.46.The obtained film was analyzed by XPS (X-ray photo electronspectroscopy) (measured in the same manner in other Examples), and themain components were confirmed to be Si and O. C in the film was 0.04mass % based on the total amount of the film.

EXAMPLE 2 (COMPARATIVE EXAMPLE)

[0054] Film forming was carried out under the same conditions as inExample 1 using a commercially available polycrystal Si target havingthe same size as in Example 1. At an applied power of 3 kW (powerdensity: 17 W/cm²), cracking was observed in the target, and thedischarge was instable. Therefore, the applied power was lowered to 1 kW(power density: 5.7 W/cm²), and film forming was carried out under thesame conditions as in Example 1 except that the voltage during the ONtime was changed to −360V, whereby the discharge was stable, and filmforming was possible. The film-forming speed per unit applied power atthat time was 70 nm/(min·kW), and the film-forming speed was 70 nm/min.The obtained film had a refractive index of 1.46, and was a filmcomposed of Si and O.

EXAMPLE 3 (COMPARATIVE EXAMPLE)

[0055] Film forming was carried out under the same conditions as inExample 1 using a SiC sintered target (CERAROI-C, trade name,manufactured by Asahi Glass Company, Limited) of the same size as inExample 1. After the film forming, the film thickness was measured bymeans of a film thickness measuring apparatus of feeler type. Thefilm-forming speed per unit applied power was 100 nm/(min·kW), and thefilm-forming speed was 300 nm/min. The refractive index of the film was1.48.

EXAMPLE 4

[0056] After preparing a molded product by a casting method as inExample 1, without carrying out the sintering in Example 1, the moldedproduct was immersed in molten Si in the same manner as in Example 1 toobtain a target comprising SiC and metallic Si as the main components.The C/Si (atomic ratio), the density, the resistivity and the thermalconductivity, of the obtained target, were equal to those in Example 1.Further, with respect to the crystal phases, only crystal phases of SiCand Si were observed as in Example 1. Further, it was confirmed that themetallic Si was present to fill spaces among SiC particles and toconstitute a continuous body.

[0057] Further, by the same method as in Example 1, the amounts of metalimpurities based on the total amount of the target were measured,whereby Al was 0.07 mass %, Fe was 0.02 mass %, Ti was 0.002 mass %, Cawas 0.008 mass %, Mg was less than 0.001 mass %, V was 0.003 mass %, Crwas less than 0.001 mass %, Mn was 0.003 mass %, and Ni was less than0.001 mass %.

[0058] This target was processed in the same manner as in Example 1, andthen film forming was carried out under the same conditions as inExample 1. After the film forming, the film thickness was measured bymeans of a film thickness measuring apparatus of feeler type. Thefilm-forming speed per unit applied power was 120 nm/(min·kW), and thefilm-forming speed was 360 nm/min. The refractive index of the film was1.46, and the C amount was 0.04 mass %.

INDUSTRIAL APPLICABILITY

[0059] The target of the present invention has durability improvedagainst cracking during film forming by sputtering, whereby a largeelectric power can be applied, and it is suitable for high speedfilm-forming of a SiO₂ film. Further, electric discharge machining ispossible, and its mechanical processing is also easy, whereby it caneasily be processed into a desired shape.

[0060] Further, according to the process for producing the target of thepresent invention, the target of the present invention can be producedis good production efficiency.

[0061] Further, according to the film-forming method of the presentinvention, a SiO₂ film having a low refractive index, can be formed at ahigh speed.

[0062] The entire disclosure of Japanese Patent Application No.11-291480 filed on Oct. 13, 1999 including specification, claims andsummary are incorporated herein by reference in its entirety.

What is claimed is:
 1. A sputtering target which comprises SiC andmetallic Si and which has an atomic ratio of C to Si of from 0.5 to 0.95and a density of from 2.75×10³ kg/M³ to 3.1×10³ kg/m³.
 2. The sputteringtarget according to claim 1, wherein the thermal conductivity of thesputtering target is at least 100 W/(m·K).
 3. A process for producing asputtering target, which comprises molding a SiC powder by a castingmethod, a press molding method or an extrusion molding method to form amolded product, and impregnating the molded product with molten Si at atemperature of from 1450° C. to 2200° C. in vacuo or in a reducedpressure non-oxidizing atmosphere, to have pores of the molded productfilled with the metallic Si, thereby to obtain the sputtering target asdefined in claim
 1. 4. A process for producing a sputtering target,which comprises molding a SiC powder by a casting method, a pressmolding method or an extrusion molding method to form a molded product,firing the molded product at a temperature of from 1450° C. to 2300° C.to obtain a sintered product, and impregnating the sintered product withmolten Si at a temperature of from 1450° C. to 2200° C. in vacuo or in areduced pressure non-oxidizing atmosphere, to have pores of the sinteredproduct filled with the metallic Si, thereby to obtain the sputteringtarget as defined in claim
 1. 5. A film-forming method which comprisesforming a film comprising SiO₂ as the main component in an atmospherecontaining an oxidizing gas by a sputtering method, characterized inthat as a sputtering target, the sputtering target as defined in claim 1is used.
 6. The film-forming method according to claim 5, wherein thesputtering method is a DC sputtering method.
 7. The film-forming methodaccording to claim 5, wherein a film comprising SiO₂ as the maincomponent and having a refractive index of at most 1.50 at a wavelengthof 633 nm, is formed.